Composite materials exhibit high strength and stiffness as well as corrosion resistant properties. In addition, their light weight is particularly advantageous when compared to similar components constructed from metals. As such, there has been increasing interest in recent years in the use of parts and assemblies constructed from fiber reinforced composite materials in industries such as, for example, the aerospace industry, where parts and assemblies having high strength to weight ratios are desired. Typical textile roving materials used as reinforcement in this regard include, without limitation, woven carbon fiber and Kevlar™ woven fiber.
One of the limiting factors to more widespread use of fiber reinforced composite parts and assemblies in, for example the aerospace industry, has been the relatively high cost of their manufacture, due in part to the fact that their production is time consuming and labor intensive (as compared to, for example, casting or stamping techniques). Moreover, the manufacture of composite parts and assemblies having sectional thicknesses greater than about 1.0 inch in any cross-sectional plane (hereinafter referenced in this specification and claims as “thicker cross-sections”) has heretofore been particularly problematic for reasons including those outlined more fully below.
Vacuum assisted resin transfer molding (VARTM) is one common prior art production method for the production of composite parts and assemblies, which method generally involves the use of a mould to enclose one or more fiber containing preforms, a means of imposing a vacuum on the mold, and resin introduced into the mold for infusion through the preform, which infusion is assisted by the draw of the vacuum. Each of the preforms is itself composed of a plurality of woven layers of the textile roving material, which layers are typically pre-adhered to one-another and pre-formed into a desired shape so as to form a core for the particular part or assembly. Generally speaking, the thicker the cross-section of the preform, the greater the number of layers therein for a given textile roving material.
While some success in manufacturing composite parts and assemblies having thicker cross-sections has been achieved through the use of methods and techniques using layers of textile roving material pre-impregnated with a selected resin (commonly and hereinafter referred to as “pre-preg sheets”) before being placed into the mould, these methods and techniques present their own set of problems including, without limitation, the very high cost of pre-preg sheets (which are typically obtained from specialized suppliers in pre-impregnated form), the high costs of handling and storing pre-preg sheets (which must typically be shipped and stored in a frozen, or semi-frozen state prior to use), and environmental and workplace safety concerns (depending upon the particular resins used for pre-impregnation, many of which may be toxic). For these and other reasons, composite molding methods and techniques utilizing pre-preg sheets have enjoyed only limited success in the construction of fiber reinforced composite parts and assemblies, and are considered by the co-inventors herein to be non-analogous prior art to the field of the present invention.
It is desirable in the production of composite parts and assemblies of consistently high strength, whether manufactured using a VARTM process, or otherwise, that the preform be substantially uniformly infused with resin during the infusion process, both between the individual textile layers of the preform and between the individual strands of fiber reinforcing material which make up each such layer. Thus, one significant limitation of non-pre-preg prior art infusion processes, including VARTM, is the difficulty of achieving the aforesaid uniform distribution of resin throughout the preform, which is required in order to substantially eliminate air voids and bubbles, and to achieve substantially complete wetting of all areas within the fiber containing preform, thereby improving adhesion of the textile layers to one another during the subsequent curing process, with resultant greater strength, consistency, and quality control for the parts and assemblies so produced. Complete wetting of the fiber containing preform becomes increasingly difficult as the cross-sectional thickness of the preform increases, particularly for the layers of the preform centrally positioned within thicker cross-sectional areas of the preform. Moreover, the time required for resin infusion of parts and assemblies increases in a non-linear fashion with cross-sectional thickness of the preform.
One prior art technique used with VARTM processes to improve resin infusion includes the use of distribution media positioned between the mould and the preform to facilitate the speed and degree of resin infiltration through the preform. However, this technique typically adds additional steps (both before and after resin infusion) and additional material to the manufacturing process and, as such, significantly increases the costs of production.
Another problem related to that of complete wetting of the preform arises from the fact that resin introduced into the mould that does not penetrate into the fibers of the preform tends to accumulate around the outer surfaces of the preform, where it does little to improve the strength of the composite part or assembly, yet adds to its weight. This phenomenon is generally referred to in the art as “race-tracking.” Thicknesses of parts or assemblies can exceed tolerable levels as a result of too much resin accumulating in regions of the preform and effectively forcing the individual plies apart. This can be problematic in many applications, especially in industries where thickness tolerances are much tighter, such as, for example, the aerospace industry. These quality control issues can result in the discarding of expensive sub-standard parts and assemblies.
Thus, for the reasons mentioned above, amongst others, it has not been practical or economical using known prior art systems or techniques to produce fiber reinforced composite parts and assemblies having thicker cross-sections and/or large thickness variations within the parts themselves. There thus exists in the prior art, amongst other things, a need to address these and other limitations, which need is increasing over time as, for example, the aerospace industry looks to increase the variety, complexity and size of composite parts and assemblies used in the construction of airplanes and spacecraft to, amongst other things, reduce weight and fuel consumption.